In one particular continuous wave electromagnetic distance measuring system in which such a phase measuring method is employed, and which is described in more detail hereinafter with reference to FIG. 1, the comparison signals of each pair of comparison signals are produced by causing a pattern signal to be propagated electromagnetically from a master station to a remote station, causing an auxiliary pattern signal to be propagated electromagnetically from the remote station to the master station, mixing the pattern signal with the auxiliary pattern signal at the master station to produce the first comparison signal, and mixing the pattern signal with the auxiliary pattern signal at the remote station to produce the second comparison signal, the comparison frequency of the two comparison signals being equal to the difference in frequency between the pattern and auxiliary pattern signals. The second comparison signal is then relayed to the master station by modulating it on a sub-carrier signal and modulating the modulated sub-carrier signal on a carrier which is used to propagate the auxiliary pattern signal to the master station. At the master station the relayed second comparison signal is detected and then phase compared with the first comparison signal. Such a system is also described in, for example, U.S. Pat. No. 2,907,999 to Wadley.
Relaying of the second comparison signal to the master station in the above manner gives rise to certain difficulties.
First, as a result of the transmission and reception of the pattern and auxiliary pattern signals, each of the comparison signals is degraded by noise. In being relayed to the master station, the second comparison signal is degraded even further by noise. This further signal to noise degradation detrimentally effects the accuracy of the phase measurements.
Second, a certain amount of cross coupling occurs between the two comparison frequency modulations in being amplified and detected through a common channel at the master station. This is usually referred to as contamination and can lead to rather unpredictable phase errors.
Third, relaying of the second comparison signal to the master station by modulation of the carrier which is used to propagate the auxiliary pattern signal to the master station reduces the transmission power available for propagating the auxiliary pattern signal. This again increases the noise content of the signals.
Other methods of relaying the phase information of the second comparison signal to the master station have been used in an attempt to alleviate one or other of the above difficulties. For example, there has been proposed a system in which the second comparison signal is frequency divided by a factor of two before being relayed to the master station, and at the master station the frequency of the signal is again, after detection, multiplied by two. This reduces contamination, but at the cost of also reducing the transmitted phase information by a factor of two and correspondingly increasing the noise content.
It is an object of the present invention to provide a novel and inventive system for dealing with contamination and signal to noise deterioration, which system at the same time leads to an elegant and economic simplification of existing continuous wave electromagnetic distance measuring systems.